124 The Plant World. 



subject to greater diurnal range of temperature than the lower 

 layers or the free air above. 



Preuss, under the title of "The boreal-alpine and pontic 

 associations of the flora of east and vest Prussia" (Ber. d. 

 Deutsch. bot. Ges.), discusses the reasons for considering that 

 a large number of boreal-alpine plants are relicts of the post- 

 glacial tundra periods, while others came in later with birches 

 and other trees from the east, rather than at a still later period, 

 as assumed by Weber. His view is based in part on geological 

 evidence, such as the fact that habitats particularly rich in 

 boreal-alpine species are chiefly found in regions of terminal 

 moraines, and in part on the character of the vegetation and of 

 the plant associations of those habitats which excludes the view 

 that a fortuitous migration of single species into the northeastern 

 lowlands could have resulted in the present composition of the 

 flora. Further evidence is drawn, also, from the biological pe- 

 culiarities of the species in question, which inhabit m.oors in 

 which the large percentage of water present and the water ca- 

 pacity of the soil result in lowering the temperature and shorten- 

 ing the period of growth under these conditions. Salix niyrtil- 

 loides, for example, a sensitive species, could not have appre- 

 ciably extended its limits in the North German lowland within 

 historical time, but is rather to be considered as a relict of an 

 epoch whose climatic conditions approached those of higher 

 latitudes. 



Raunkiaer has reported in the Memoires of the Denmark 

 Academy the results of his studies of the Strand vegetation of 

 some of the Danish West Indian Islands as compared with that 

 of similarly situated land in Denmark. He lays special em- 

 phasis on temperature and on humidity originating from pre- 

 cipitation in determining the character of strand vegetation. 



Something over a dozen years ago a cruciferous plant was 

 found growing wild at Lindau, Germany, which resembled 

 Capsella bursa-pasioris but differs from it in various characters to 

 an extent that led Solms-Laubach, after making trial cultures, to 



